This invention relates to a process for production of expanded pellets from a thermoplastic elastomer having an elongation at break of more than 100% measured to DIN EN ISO 527-2. Expanded pellets of thermoplastic elastomers having an elongation at break of more than 100% measured to DIN EN ISO 527-2, have elastic and tribological properties and so are useful in a wide variety of applications. Examples of uses for corresponding expanded pellets include reusable gymnastics mats, body protectors, trim elements in automobile building, sound and vibration absorbers, packaging or in shoe soles. High elasticity and good homogeneity on the part of the pellets are of decisive importance for all these sectors.
Foamed materials, including bead foams in particular, are long known and have been extensively described in the literature, for example in Ullmann's “Enzyklopädie der technischem Chemie”, 4th edition, volume 20, p. 416 ff.
WO 2007/082838 discloses a process for production of expanded thermoplastic polyurethane comprising a blowing agent. A first step of the process comprises extruding a thermoplastic polyurethane into pellets. The pellets are impregnated with a blowing agent in an aqueous suspension under pressure in a second step and expanded in a third step. In a further embodiment of the process, the thermoplastic polyurethane is melted in an extruder together with a blowing agent and the melt is pelletized without a device to prevent foaming. Volatile organic compounds are used as blowing agents in the production via extrusion.
EP-A 0 664 197 discloses the production of expanded thermoplastic elastomers using water as blowing agent in an endeavour to avoid organic blowing agents. An alternative process for producing foams from thermoplastic elastomers by use of carbon dioxide and nitrogen as blowing agents is known, for example from WO 2004/018551. A further process for producing expanded thermoplastic elastomers which recurs to the foam production process described in WO 2004/018551 is also disclosed in WO 2007/044123.
None of the documents known from the prior art, however, discloses that the process described can also be used to produce expanded pellets having an uninterrupted skin.
The use of an extrusion process to produce expanded pellets of TPU permits continuous production and hence rapid processing of a variety of hardnesses and also the rapid switch between further properties, for example the color of the expanded beads produced.
Yet there is a problem with the direct production of expanded pellets via extrusion in that the beads expand without an uninterrupted skin forming in the process and the expanded beads collapse, making it impossible to produce beads of low bulk density. It is similarly disadvantageous that the blowing agents used are flammable and so are difficult to process because of an ever present risk of explosion. Furthermore, the expanded pellets produced have to be stored until the flammable blowing agent used has volatilized before they can be shipped out.